1. Field of the Invention
The present invention is directed generally to a method and apparatus for timing and frequency generation and, more particularly, to a method and apparatus for sequentially synchronized timing and frequency generation in a communication network.
2. Description of the Background
In a wireless access system, the synchronization of time and frequency of transmissions are of paramount importance. Transmissions that are synchronized and share a known time and frequency reference provide improved system acquisition, simplified mobile station searching, improved handoff reliability, improved handset standby time, and facilitated location and position searching.
The majority of IS-95and Code Division Multiple Access (CDMA) deployments operate on GPS (Global Positioning System) time in order to gain a universal time reference for synchronization, and in order to gain the benefits which follow from synchronization. However, an increasing number of network operators find dependence on the GPS undesirable, and the need to make a GPS measurement at each base station adds increased cost and additional time consumption to the wireless network.
Therefore, the need exists for a wireless communication network that provides synchronization to a known time reference, while providing an alternative time reference to GPS time. A further need exists for a wireless communication network that provides simplified synchronization of all base stations in the network to a known frequency reference.
The present invention is directed to a synchronized timing and frequency generator for a communication network. The synchronized timing and frequency generator includes a parent station which maintains system time and frequency values, a first time/frequency transfer unit which receives the system time value from the first parent (master) station and generates corrected system time and frequency values, and a first child (slave) station to which the first time/frequency transfer unit directly communicates the corrected system time and frequency values. This hierarchy of parent station-time transfer unit-child station-parent station may be repeated for as many stations as are deployed in a given wireless network. The corrected system time value may be generated using an adjustor which advances or retards a local free running clock at a child station.
In a preferred embodiment, the present invention is directed to a first time/frequency transfer unit coupled to a first child base station in a sequential time and frequency synchronization system. The first time/frequency transfer unit includes a receiver which acquires a pilot signal set, a demodulator which demodulates a SYNC message of a SYNC channel signal from the parent base station, determines the unit system time from the SYNC message, then advances the unit system time by a predetermined amount corresponding to the propagation delay between the parent station and the first time/frequency transfer unit in order to obtain absolute system time. The first time/frequency transfer unit then uses the absolute system time to generate a periodic pulse train with well defined edges used for controlling the timing of signals sent from the first child station. In a preferred embodiment, the period of the pulse train is an integer multiple of 1 second, and the SYNC message and SYNC channel correspond to the SYNC message and SYNC channel defined in the IS-95A standard. The generator used at the first time/frequency transfer unit to generate the periodic pulse train may include an adjustor which measures a time difference between the output of a free running local clock at the first child base station and the absolute system time determined by the first time/transfer unit, and then retards the output of the free running clock so as to synchronize the output of the free running clock with the absolute system time determined by the first time transfer unit.
Once the timing of the first child base station has been synchronized as described above, the first child base station uses the absolute system determined by the first time/frequency transfer unit to control the time synchronization of signals sent from the first child base station. In addition, the first child base station begins transmitting its own SYNC message on its SYNC channel in accordance with the absolute system time (as determined by the first time/frequency transfer unit) to a further child base station, thereby causing the first child base station to become a further (second) parent base station. The process described above is then repeated by a second time/frequency transfer unit coupled to the second child base station in order to synchronize the absolute system time used by the second child station with that of the first and second parent stations. The process is then preferably repeated for all further base stations in a communication system, thereby resulting in all such base stations being synchronized to a common absolute system time.
In the preferred embodiment, the time/frequency transfer units coupled to the child base stations are also used to sequentially establish frequency synchronization throughout the system of base stations. As mentioned above, the first time/frequency transfer unit includes a receiver which acquires a pilot signal from the parent base station. The pilot signal is transmitted from the first parent station at a first center frequency. The first time/frequency transfer unit coupled to the first child station receives the pilot signal from said first parent station and generates a corrected system frequency value by translating the center frequency of the received pilot signal to a predetermined reference center frequency (e.g., 10 MHz). The corrected system frequency value from the first frequency transfer unit is then communicated to the first child station and used to synchronize the frequency of the first child station with that of the parent station. Once the frequency of the first child base station has been synchronized as described above, the first child base station begins transmitting the translated pilot signal to a further child base station, thereby causing the first child base station to become a further (second) parent base station. The process described above is then repeated by a second time/frequency transfer unit coupled to the second child base station in order to synchronize the frequency used by the second child station with that of the first and second parent stations. The process is then preferably repeated for all further base stations in a communication system, thereby resulting in all such base stations being synchronized in frequency. In a preferred embodiment, the: pilot signals and pilot channels used for this aspect of the invention correspond to the pilot signals and channels defined in the IS-95A standard.
The present invention also includes a method of synchronizing timing generation in a communication network. The method includes receiving a system time value from a parent station at a time transfer unit, generating a corrected system time value at the time transfer unit, and transferring the corrected time value to a child station. The communicating may include comparing a free running local clock at said child station to the generated corrected system time value, and adjusting the free running local clock at the child station in accordance with the comparing.
The present invention solves problems experienced with the prior art because the present invention provides synchronization to common time and frequency references, without dependence on GPS timing as a reference at each base station. These and other advantages and benefits of the present invention will become apparent from the detailed description of the invention hereinbelow.